Twenty Pounds was a lot of money during the era of the Old Note issues.
The notes are rare, and seldom offered.
When I started collecting these around 20 years ago, few notes of this denomination had been recorded, other than specimens and proofs.

They do turn up though, and several collections have been sold over the past decade which had £20 notes in them.

So far, £20 notes of the following banks have been seen:
Bank of Ireland
National
Ulster
Belfast Banking Company
Provincial Bank

Paper Money of Ireland documents an example of an issued Northern Bank £20 dated 1 Feb 1918 (Type NR 41). This type of note appears to be very rare. Curiously, old £50 and £100 notes of the Northern Bank overprinted and issued in 1929 are more often seen.